1. Field of the Invention
The present invention relates to bending glass sheets in pairs to sharp curvatures including a pair of sharp bends extending across the width of the glass sheet near the longitudinal extremities thereof. These bends are popularly known as double V-bends.
Glass sheets have been bent by the sag bending method by mounting glass sheets on outline molds of the skeleton type and introducing the glass sheet laden molds into a furnace where the glass sheets are heated to a deformation temperature and sagged to conform to the upper surface of the mold. Glass sheets are also bent to sharp curvatures by combining with the overall heat in an enclosed furnace, localized heat applied along the lines to be sharply bent either from elongated heating elements closely spaced to the lines of sharp bending of the glass or along lines of electroconductive material applied to the glass surface along lines of sharp bending.
In bending glass sheets in pairs, it is important to provide greater heat to the upper surface of the pair of sheets mounted for bending than to the lower surface so that the sheets bend in unison. If too great a proportion of the total heat is applied to the bottom of the pair, gaps form between the sheets which make the task of laminating the bent sheets to one another difficult.
In the past, electrical heating elements incorporated in molds for providing elongated lines of sharp bending in the glass have been either rigid rods or electroconductive ribbons attached to some structure on the bending molds. This has caused considerable difficulty in storing unused molds when other patterns are in production.
Sharp bends have been produced in glass by applying electroconductive materials which remain on the glass surface after the sheet is bent to the shape desired. Frequently, the material must be removed and removing the electroconductive materials presents problems.
Sharp bends have also been produced by applying electroconductive heating elements or mounting means therefor directly to at least the upper surface of glass sheets to be shaped. This method has several drawbacks including the fact that elements that contact the glass surfaces while the latter are hot tend to mark the glass.
2. Description of the Prior Art
In recent years, several patents have issued relating to the bending of glass sheets to sharp curvatures. U.S. Pat. No. 3,762,903 to Harold E. Hamilton; U.S. Pat. No. 3,762,904 to Harold E. Hamilton, Robert E. Bamford and Paul Pastorek; U.S. Pat. No. 3,879,184 to Harold E. Hamilton and Ivan L. Soreghy; an U.S. Pat. No. 3,865,680 to Thomas J. Reese and Harry S. Koontz disclose glass bending methods and apparatus employing lines of electroconductive material that are applied directly to the glass surface to help form the sharp bend along lines of sharp bending
U.S. Pat. No. 3,795,50l to Robert A. Jansson and Thomas J. Reese; U.S. Pat. No. 3,795,570 to Robert A. Jansson and Dean L. Thomas; and U.S. Pat. No. 3,847,586 to Thomas J. Reese, George R. Claassen and Melvin W. Tobin disclose gravity sag molds that incorporate flexible metal ribbons as auxiliary heating elements that augment the heating of glass sheets along elongated lines of sharp curvature.
U.S. Pat. No. 2,111,392 to Henry J. Galey; U.S. Pat. No. 2,176,999 to Robert A. Miller; U.S. Pat. No. 2,215,228 to James G. Oliver; U.S. Pat. No. 2,999,338 to Ronald L. Richardson; and U.S. Pat. No. 3,248,195 to James S. Golightly and Harold E. McKelvey disclose bending apparatus having electroconductive radiant heaters other than elongated flexible ribbons that apply sharp localized bends to a glass sheet along an elongated axis thereof. However, these patents show apparatus that support the heating elements in such positions that makes it difficult and time consuming to load or unload a glass sheet from the mold. Furthermore, many of these patents provide heaters that are permanently attached to the mold so that it is difficult to store molds that are not being used in production without requiring a great deal of space for mold storing purposes.
It has also been proposed to support one or more heating ribbons below the position occupied by a single glass sheet during bending in order to facilitate loading and unloading of the glass sheet so as to avoid the time delay needed to apply and remove the ribbons for each bending cycle. Such positioning of heating ribbons is impractical when two glass sheets are bent simultaneously in pairs because, unless the additional heat from the ribbons is applied from above the pair of glass sheets, the sheets separate during bending, thus making it difficult to laminate the bent glass sheets.